Introduction
During chewing, restorations rub against opposing teeth, and both surfaces slowly lose substance. Wear resistance (erasability) is the ability of a material to resist this loss. Abrasiveness is its ability to wear down the opposing teeth.
A good material resists wear about as well as enamel, without damaging what it bites against.
What affects wear
- Surface smoothness
- Structural homogeneity of the material
- Corrosion and fatigue
- Chewing load
- Saliva, which acts as a lubricant
- Type of food and bad habits
Normal physiological wear is about 15 to 30 µm a year. In people who eat root crops mixed with sand, it can reach about 150 µm a year.
Mechanisms of wear
These mechanisms act at the same time in the mouth, in differing degrees.
- Two-body abrasion: leaves many roughly parallel lines. Abrasive tools work this way.
- Three-body abrasion: polishing with a brush and paste is the classic example.
- Adhesion: friction joins the surfaces by cold welding, and material passes between them.
- Fatigue: repeated loads below the maximum strength cause microcracks that join and release a fragment.
- Tribochemical: chemical attack softens the surface, wear removes that layer, and fresh material is exposed to attack.